The present invention relates to a magnet sorting apparatus for measuring soft and hard magnets cut into optional shapes and sizes and automatically sorting the magnets in accordance with the result of the measurement.
Commercially available magnets have a variety of magnetic surface flux densities in their magnetized state due to subtle differences in the magnet materials. When unsorted magnets are employed in an appliance such as headphone, actuator for compact disc player (CDP) and the like, it contributes to causing a degradation in the quality of the appliance. Therefore, it is required to use magnets exhibiting uniform properties.
FIG. 1 is a circuit diagram illustrating a circuit for measuring a characteristic of a magnetic material.
In the illustrated circuit, voltages V.sub.1 and V.sub.2 induced across coils N.sub.1 and N.sub.2 can be derived in accordance with the following equations: ##EQU1##
In case of N.sub.1 =N.sub.2, the above equation (3) can be expressed as follows: ##EQU2##
By integrating both parts of the just above equation for t, the following equation is established: ##EQU3##
That is, the quantitative magnetization M of a magnet surrounded by the coil N.sub.2 can be derived by integrating the signal V.sub.0.
By arranging the equation (1) after integrating for t, the following equation is established: ##EQU4##
That is, the intensity H of an external magnetic field per field can be derived by integrating V.sub.1.
Since the equation (2) corresponds to a state prior to a pulse application, dH/dt is zero (dH/dt=0).
Accordingly, the following equation can be established: ##EQU5##
By M-H plotting the results of the equations (4) and (5), an iHc curve shown in FIG. 2 can be obtained.
Since B=H+M, the value of B can be derived by summing the results of the equations (4) and (5). By B-H plotting the result of the summing calculation and the result of the equation (4), a bHc curve shown in FIG. 2 can be obtained.
In the above equations,
V.sub.1, V.sub.2 : voltages induced in respective coils;
V.sub.0 : V.sub.1 and V.sub.2;
A: cross-sectional area of each coil;
Am: cross-sectional area of magnet;
N.sub.1, N.sub.2 : numbers of turns of respective coils;
M: quantitative magnetization of magnet;
H: intensity of external magnetic field;
t: time;
n: point of time when measurements are completed; and
M.sub.0 (J.sub.0): initial quantitative magnetization of magnet for a residual magnetic flux density of B.sub.r.
By referring to the equations, it can be found that in a general magnetic substance, a magnetic hysteresis phenomenon occurs between a magnetic field externally applied and a quantitative magnetization (spontaneous magnetization) of a magnet.
FIG. 2 is a graph of a hysteresis loop, illustrating only curves of the second quadrant thereof.
By analyzing a characteristic of the second quadrant of the hysteresis loop corresponding to the case wherein a magnetic field externally applied is opposite in direction to line of magnetic force generated from the magnet, the characteristic of the magnet can be found (in hard magnets, more advantageous results may be obtained).
In FIG. 2, the iHc curve shows a relation of "the quantitative magnetization of the magnet itself" to an external magnetic field H whereas the bHc curve shows a characteristic resulted from a consideration of both the characteristic of the magnet itself and the magnetic field externally applied with respect to the external magnetic field H.
The magnetic flux density of the magnet is determined, depending on the shape of the magnet and the environment around the magnet.
Lines determined after taking into consideration the above-mentioned are permeance lines (hereinafter, referred to as "P lines." These lines are shown as lines P.sub.1 and P.sub.2 in FIG. 2.
The density of magnetic flux generated from the magnet corresponds to the value of B at a point of intersection between the bHc curve and the P line. For example, if the line P.sub.1 is assumed as the P line at a bare magnet state, the density of magnetic flux generated from the magnet corresponds to B.sub.1.
For employing a magnet in an appliance such as a motor or speaker, generally, a yoke is attached to the magnet for getting the P line closer to the B-axis and thereby increasing the surface magnetic flux density. Assuming the line P.sub.2 of FIG. 2 as the P line in this case, the density of magnetic flux generated from the magnet corresponds to B.sub.2. That is, the magnetic flux density is increased from B.sub.1 to B.sub.2.
Values of H at points of intersection between the iHc curve and the H-axis and between the bHc curve and the H-axis in FIG. 2 are indicative of coercive forces for the iHc curve and bHc curve, respectively. These coercive forces are indicated by iHc and bHc, respectively.
Conventionally, evaluation and sorting of magnets exhibiting uniform characteristics are achieved by manually performing measurements of the surface magnetic flux density and the magnetic flux for each magnet at a bare magnet state or at a yoke-attached state by use of a measuring instrument, and then determining, as the total characteristic values of each magnet, characteristic values at one or two points on the hysteresis loop, which points correspond to the measured values.
Since the above-mentioned conventional method carries out the evaluation and sorting of a magnet using only one or two points on the hysteresis loop for the magnet, the surface magnetic density of this magnet may be considerably different from the surface magnetic density B.sub.2. This is because only the value B.sub.1 on the P line is measured in accordance with the conventional method.
Precise measuring instruments are commercially available, however, these measuring instruments can not be used for quality control because of their low processing speed (one or two days are taken for a measurement) and a low applicable magnetic field of, for example, 20 KOe. The sorting work after completion of the measurement is also manually carried out. As a result, the productivity associated with the sorting is degraded.